1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including an electrophotographic copy machine, an electrophotographic printer (for example, LED printer or laser beam printer), and an electrophotographic facsimile apparatus.
2. Description of the Related Art
FIG. 10A illustrates a schematic structure illustrating a charge bias circuit of a conventional image forming apparatus. A charge DC bias circuit part (hereinafter, referred to as charge bias application circuit unit) 1201 includes a voltage set circuit part (unit) 1202, a transformer drive circuit part 1203, a high-voltage transformer part (unit) 1204, and a feedback circuit part (unit) 1205. The voltage set circuit part (unit) 1202 may change a voltage value set based on an input PWM signal. The transformer drive circuit part (unit) 1203 drives the high-voltage transformer part (unit) 1204. The feedback circuit part (unit) 1205 uses a resistor R1201 to detect a voltage value applied to a charge roller 1206 (load) which is a charging member, and transfers the detected voltage value as an analog value to the voltage set circuit part (unit) 1202. The voltage set circuit part (unit) 1202 controls to apply a constant voltage (voltage indicated by PWM signal) to the charge roller 1206 which is the charging member based on a value of the PWM signal and a feedback value. When an applied voltage is controlled using such a structure, a constant voltage value may be applied to the charge roller 1206 (see, for example, Japanese Patent Application Laid-Open No. H06-003932).
However, a voltage for starting discharge between the charging member (charge roller) and an image bearing member changes depending on a circumstance temperature and humidity in the image forming apparatus or a film thickness of a photosensitive drum (hereinafter, simply referred to as drum film thickness). Therefore, as illustrated in FIG. 10B, even when control is performed to apply a predetermined voltage (controlled PWM value), a variation in potential on the photosensitive drum is caused by temperature-humidity (such as low temperature and low humidity (L/L) or high temperature and high humidity (H/H)). The variation causes a change in image density. In order to correct the change in image density, it is necessary to provide a density detection sensor for detecting an image density and a temperature-humidity sensor for detecting a temperature and a humidity in the image forming apparatus, and control the image density based on a result obtained by detection by the density detection sensor. When the density detection sensor and the temperature-humidity sensor are provided in the image forming apparatus, an apparatus cost may increase. When the density detection sensor and the temperature-humidity sensor are not provided, it is difficult to adequately correct the change in image density.